The ALTER PARTITION and SELECT seemed to be deadlocked
when having innodb_thread_concurrency = 1.

Problem was that there was unreleased latches
in the ALTER PARTITION thread which was needed
by the SELECT thread to be able to continue.

Solution was to release the latches by commit 
before requesting upgrade to exclusive MDL lock.

Updated according to reviewers comments (3).

mysql-test/r/partition_innodb.result:
  updated test result
mysql-test/t/partition_innodb.test:
  added test
sql/sql_partition.cc:
  Moved implicit commit into mysql_change_partition
  so that if latches are taken, they are always released
  before waiting on exclusive lock.
sql/sql_table.cc:
  refactored the code to prepare and commit
  around copy_data_between_tables, to be able
  to reuse it in mysql_change_partitions
sql/sql_table.h:
  exporting mysql_trans_prepare/commit_alter_copy_data